This invention relates to a heat-sensitive record material and particularly to a heat-sensitive record material which is adapted for a high speed and unremitting recording so that it may find its usefulness as a recording medium for information machines and instruments such as facsimiles, electronic computers and telex machines.
There is known a heat-sensitive record material comprising a base sheet having a color developing layer which includes finely divided particles of one of electron donating color forming materials (hereinafter referred to as "color former") such as triphenylmethane compounds, fluoran compounds, phenothiazine compounds, auramine compounds and spiropyran compounds and finely divided particles of one of organic acceptors such as phenolic compounds, aromatic carboxylic acids and their polyvalent metal salts and/or one of inorganic acceptors such as activated clay, acid clay, attapulgite, aluminum silicate and talc. In such the heat-sensitive record material like this the above mentioned two kinds of particles are, when at least one of them is melted or sublimated at an elevated temperature, brought into intimate contact with each other to develop a color.
One of the most typical heat transmission systems for developing a color image on the above mentioned heat-sensitive record material is to transfer heat to the heat-sensitive record material through the utilization of a thermal head having a number of electric resistance heating elements through which Joule heat produced by electric current pulses in response to signals to be recorded can be transmitted to the surface of the heat-sensitive record material when the thermal head is into close contact with the heat-sensitive record material. An inevitable trouble with this type of heat transmission is the fact that the color developing material which is in a melted state when heated is transferred and adhered as smudges or tailings to the thermal head. The smudges or tailings adhered to the thermal head gradually and steadily grow during a continuous recording operation with the result that the thermal conductivity from the thermal head to the heat-sensitive record material is reduced. This is apparently disadvantageous since clear and distinct color images can never be expected at a high speed recording.
Another trouble involved with the above mentioned heat transmission system is the fact that the heat-sensitive record material is liable to be stuck in contact with the thermal head to which heat is transferred. This "sticking" prevents the heat-sensitive record material from being smoothly fed in a constant speed. This after results in irregularly leaving space between the typing lines, deforming the record images, interrupting the recording and damaging the thermal head.
With an attempt to avoid the above mentioned disadvantage it has been proposed in Japan Kokai (Laid-Open Patent Publication) No. 33,832 of 1973 and U.S. Pat. No. 3,859,112 to add to the color developing layer conventioned inorganic pigments such as clay, talc, calcium carbonate and titanium oxide. In order to substantially avoid adhesion of smudges or tailings, however those conventional inorganic pigments must be used in such an extremely large amount that the image density is lowered.
Another attempt to prevent adhesion of smudges or tailings to the thermal head and "sticking" between the heat-sensitive record material and the thermal head is to increase the amount of the binder used in the color developing layer. This attempt has also involved the lowering of the image density.
A further attempt made by the inventor et al to avoid the above mentioned disadvantages is to add to the color developing layer of a heat-sensitive record material a pigment having a large oil absorption which is capable of effectively absorbing color developing materials, as disclosed in Japan Kokai (Laid-Open Patent Publication) No. 86,229 of 1978 and U.S. Pat. No. 4,168,845. In this manner the thermal head can be protected from smudges without sacrificing the record image density and recording operation can be carried out at a relatively high speed without stopping for a relatively long time.
However, recently a further high speed recording has been required in information machines and instruments such as facsimiles, electronic computers and telex machines in which the heat-sensitive record material is used as a recording medium. This also requires that the printing time of the thermal head on the heat-sensitive record material for recording is extremely shortened. In such a extremely high speed recording like this a satisfactory continuous recording can not be expected even with the utilization of the above mentioned pigment having a large oil absorption. Increasing the amount of the pigment having a large oil absorption involves a disadvantages of lowering the image density.
The primary object of the invention is to provide an improved heat-sensitive record material which can prevent the thermal head from being smudged and stuck without sacrificing the record image density and the recording sensitivity.
Another object of its invention is to provide an improved heat-sensitive record material which can satisfactorily meet the requirements of recording machines and implements in which continuous recording for a long time is carried out at an extremely high speed.
Other objects and advantages of the invention will be apparent from the following detailed description.